1. Field of the Invention
The present invention relates to a method of depositing rare earth oxide thin films. In particular, the invention concerns a method of growing yttrium, gadolinium and lanthanum oxide thin films by Atomic Layer Deposition (referred to as ALD hereinafter).
ALD has previously been known as Atomic Layer Epitaxy (ALE) and later more specifically as Atomic Layer Chemical Vapor Deposition (Atomic Layer CVD™ and ALCVD™) process which are trademarks of ASMI®. ALD has been adopted as a general name of the method to avoid possible confusion when discussing about polycrystalline and amorphous thin films. Other names used in the literature for ALD are digital epitaxy, digital layer epitaxy (DLE), atomic layer growth, atomic layer processing, deposition layer by layer, sequential CVD, cycle CVD, cyclic CVD and pulsed CVD. The ALD method is based on sequential self-saturated surface reactions.
2. Description of the Related Art
According to N. N. Greenwood et al. (Chemistry of the Elements, 1st edition, Pergamon Press Ltd., U.K., 1986, page 1423) “rare-earth elements” comprise Sc, Y, La and lanthanide series from Ce to Lu. These elements belong to metals. Oxides of rare earth elements are called rare earth oxides, REOx. A general symbol Ln is often used in the literature to refer to the fourteen lanthanide elements cerium to lutetium inclusive. Sometimes lanthanides are also called as “lanthanons” or “lanthanoids”. REOx thin films have potential applications in compound semiconductor and/or silicon based microelectronics. Compound semiconductors have several advantages compared to silicon. Especially, electron mobility is remarkably higher in compound semiconductors than in silicon. Therefore, it is possible to produce faster components from compound semiconductors. Furthermore, compound semiconductors are efficient light emitters enabling easy integration to light emitting components.
One remarkable problem related to compound semiconductors is lack of passivating and dielectric native oxide (compare SiO2 on silicon). So far, no other oxide has worked successfully in compound semiconductor MOSFETs (metal-oxide-semiconductor filed effect transistors) resulting in low space densities on semiconductor-insulator interface. In 1999 M. Hong et al. (Science 283 (1999) 1897) showed that the requirements for MOSFET insulator could be fulfilled by growing an epitaxial layer of Gd2O3 on GaAs surface. At the same time, it was disclosed that other rare earth oxides would work similarly.
In silicon based integrated circuits, REOx is a potential material to replace SiO2 as the gate oxide in MOSFETs. To reach the required capacitance, the thickness of SiO2 should be reduced to a level where the tunneling of fuel charge through the insulator becomes evident. Therefore, to avoid the problem, SiO2 must be replaced with material that has higher dielectric constant than SiO2, is thermally stable in contact with silicon and which can be formed On silicon in a controlled manner. SiO2 layer or electrically active defects must not be formed at the interface between silicon and the insulator and the thickness of the dielectric layer should be carefully controlled.
SiO2 may be formed to an interface between silicon and the dielectric because of interaction between silicon and the dielectric or because of oxidation that takes place during depositing the dielectric layer or during a high-temperature anneal. Favorably, the deposition process of the dielectric is carried out at low temperature, where the formation of SiO2 is kinetically hindered.
REOx are thermodynamically stable when contacted with silicon oxide. Therefore, REOx are suitable dielectrics on silicon.
Yttrium oxide and lanthanum oxide are interesting thin film materials especially in the point of view of the semiconductor industry. Y2O3 thin films have been produced by many different processes, whereas considerably less research has been focused on La2O3. The production of Y2O3 and La2O3 thin films by different methods and their applications are surveyed in the literature. The production methods of the thin films are roughly divided into physical and chemical processes including both gas phase and liquid phase methods.
Because of the physical properties of Y2O3 such as the crystallographic stability up to 2330° C., high mechanical strength, high dielectric constant and the value of the refractive index, Y2O3 thin films have many potential applications (Gabordiaud, R. J. et al., Appl. Phys. A 71 (2000) 675-680). Especially interesting feature, from point of view of electronic applications, is quite good compatibility of the lattice constant of Y2O3 with silicon: a(Y2O3)=10.60 Å and a(Si) 2=10.86 Å (Cho M.-H. et al., J. Appl. Phys. 85 (1999) 2909-291).
Perhaps the most important application of Y2O3 thin films is to use them in transistors as an alternative gate oxide material having a high dielectric constant. The significance and use of an alternative gate oxide material is described in more detail later. Another application for the dielectric thin film in silicon technology is capacitor dielectric in DRAM-memories (dynamic random-access memory) (Kingon et al, Nature 406 (2000) 1032-1038).
Y2O3 thin films have been used as buffer layers for example for ferroelectrics and new high temperature superconductors. Y2O3 is also an important material in optical applications. For example, Y2O3 thin films have been used as dielectric layer in electroluminescent displays. Y2O3 matrix activated with europium has red luminescence and can be used, e.g., in fluorescent lamps and CRT tubes. Y2O3 has proved to be useful as a protective coating.
Despite the interesting properties of La2O3, possible applications of the La2O3 thin films have been studied rather little for the time being. La2O3 thin films have been used as optical and protective coatings. La2O3 coatings have been used also in gas sensor and catalytic applications. However, because of the high dielectric constant and compatibility with silicon, La2O3 is a possible gate oxide material in the future. Promising results have been recently reported by replacing SiO2 with La2O3 as a gate oxide.
Continuous decrease of the size of the electronic components has set severe restrictions on the performance of the SiO2 gate oxide. Thickness of the gate oxide approaches the quantum tunneling junction of 10 Å for SiO2. An alternative solution is to find a new dielectric material having a dielectric constant κ essentially higher than 3.9 for SiO2. The substituting alternative dielectric material has to be thermally stable at temperatures even over 1000 K, due to high-temperature anneals required in modern silicon processes. Equivalent thickness of SiO2 teq has to be below 15 Å. Equivalent thickness of SiO2 is defined with an equation:
                              t          eq                =                              t            ox                    ⁡                      (                                          κ                                  SiO                  2                                                            κ                ox                                      )                                              (        1        )            wherein tox is the actual thickness of the alternative dielectric material, κSiO2 is the dielectric constant 3.9 of SiO2 and κox is the dielectric constant of the alternative dielectric material.
The principle and applications of atomic layer deposition (ALD) are described extensively below. Since the deposition temperature is considerably high in most thin film deposition methods, ALD opens new possibilities to use low deposition temperature. According to literature, the Y2O3 thin films have been deposited for time being only by using Y(thd)3 or derivatives thereof as the ALD source material.
In the atomic layer deposition method the principle is to feed source materials by alternately pulsing them into the reactor space. During each source material pulse excess source material is present in the gas phase of the reaction space in order to saturate the substrate surface. The excess of the source material that is physi-sorbed on a surface or which is in a gas phase inside the reactor is purged away with an inert gas flow during the time interval between different source chemical pulses. In an ideal case only one atom layer or a specific fraction thereof is chemisorbed onto the substrate. Another source material pulsed subsequently reacts with the chemisorbed layer. The growth of film is controlled by the surface reactions, so the duration of the source material pulses does not need to be controlled as precisely as in other CVD methods.
In an ideal case, a single atomic or molecular layer is grown during one source material pulse, but in practice the growth rate remains considerably lower. Reason to this is most commonly steric hindrances due to the size of the source material molecules.
An ALD type process is controlled by surface reactions, which can often be controlled by process temperatures and gas flowing rates. An appropriate temperature range is called ALD (or ALE) window. Parameters preventing the ALD growth outside the ALD window are shown in FIG. 1 (Niinistö et al., Proc. Int. Semicond. Conf. (2000) 33-42).
Mölsä et al. (Adv. Mater. Opt. Electron. 4 (1994) 389-400) have grown Y2O3 thin films in a flow-type ALD reactor using Y(thd)3 and oxygen or ozone as the source materials. The aim of the study was to produce a Y2O3 buffer layer for high temperature superconductor films. The effect of the substrate material, pressure and pulsing time on the properties of the thin film was examined. The tested growth temperature range was from 425 to 600° C., which is too high for many applications. The growth rate was determined to be about 0.8 Å/cycle, but the growth rate was observed to increase with the increasing temperature. This indicates the lack of so called ALD window, which was the basic starting point for further studies of Putkonen et al. (Chem. Vap. Deposition 7 (2001) 44-50).
Putkonen et al. studied the ALD deposition of the Y2O3 thin films in the temperature range of 200-425° C. by using Y(thd)3−, Y(thd)3(bipyridyl)- or Y(thd)3(1,10-fenantroline) compounds as the metal source and ozone as the oxygen source. A constant growth rate of 0.22-0.23 Å/cycle was observed in the temperature range of 250-350° C. for all source materials. The ALD window representing the observed controlled growth is shown in FIG. 2. This temperature range is considerably lower than temperatures used previously in CVD depositions of the Y2O3 thin films. However, the growth rate remained impractically low. Also the hydrogen and carbon impurity levels were rather high. FIG. 2 depicts carbon and hydrogen content as a function of the deposition temperature.
Crystallinity and orientation of the films depended on the deposition temperature. Crystallinity increased strongly as the deposition temperature was elevated over 375° C. The films grown onto Si(100) and soda lime glass substrates at the deposition temperature of 350° C. were polycrystalline with (400) and (440) reflections being dominant (FIG. 3).
Despite the application possibilities of La2O3, only few articles have been published in literature on the deposition of the La2O3 thin films. Electron spray evaporation, different thermal vaporizing processes, pulsating laser deposition and atom spray deposition amongst physical methods have been used. Only pyrolysis, CVD, and ALD (Seim H. et al., Appl. Surf. Sci. 112 (1997) 243-250, Seim H. et al., J. Mater, Chem. 7 (1997) 449-454 and Nieminen M. et al. Appl. Surf. Sci., in press) present the chemical deposition methods.
Nieminen et al. studied ALD deposition of La2O3 using La(thd)3 as a lanthanum source in order to find an ALD window. A temperature range from 180 to 425° C. was examined. Si(100) and soda lime glass were used as substrates. The growth rate of the films as a function of temperature is shown in FIG. 4. The pulsing time for La(thd)3 was 0.8 and for ozone 2 s. A constant growth rate of 0.36 Å/cycle was detected for the temperature range from 225 to 275° C. Therefore a self-controlling deposition process typical to ALD was observed at this temperature range. X-ray diffraction (XRD) measurements on films showed to be comparable with the data presented by Seim et al. Stoichiometry and carbon content of the films were determined by TOF-ERDA (Time-of-Flight Elastic Recoil Detection Analysis) and RBS (Rutherford Backscattering Spectrometry). The carbon content depended on the deposition temperature (FIG. 4). However, in the range of the self controlled growth the elemental contents correspond to those of La2O2CO3, indicating very poor quality of the resulted film because of the carbonate incorporation. Bending vibrations were observed in the IR-measurements of films grown over 350° C. because of the hydroxyl groups present in the film.